There is no hard limit. The only policy is that sufficiently unrelated questions should be asked in separate posts.
What does "sufficiently unrelated" mean here? Basically, questions should only be grouped together if you can reasonably imagine someone else who has one of those questions in mind would likely be simultaneously wondering about the others.
If someone asks a question about neutrino absorption cross sections in lead, it is reasonable that they ask about the cross sections for each of the three generations of neutrinos. However, if some student just lists five thoughts that occurred to them while taking intro EM this past semester, that would probably warrant splitting into separate posts.
I find that a good discriminant is whether or not the questions are (or could easily have been) formatted into a bullet-point list. A list of questions is unlikely to have a strong conceptual underpinning uniting them, and is probably a sign that the questions are not sufficiently alike to be kept together.
On the other hand, multiple questions that flow with the problem statement are often good - they tend to ask about the same underlying physics from different angles. For instance, one of my questions has 5 question marks spread throughout the body, but they are all driving at the same underlying issue. Answering just one would be insufficient for settling the matter, and it would be difficult to answer one without simultaneously answering the others.